"An inner core of solid iron is surrounded by an outer core of molten iron. They rotate at different rates, and the interaction between the regions creates what scientists call a 'hydromagnetic dynamo.'"

From this web site:    Interesting stuff.

According to this, the shift of the magnetic poles doesn't happen overnight as some folks contend, not is it cataclysmic, as the "let's all panic!!" folks love so much, it "migrates" over a period of possibly thousands of years.

Don Firth